Kean University Geology 3263
Structural Geology
Field Trip 2
Geology of Wissahickon Creek, near Chestnut Hill, Pennsylvania
This area shows us metamorphosed nearearc Flysch and associated amphibolites from
Ophiolite sequences. Locally there are migmatites and intrusive granites. The
Wissahickon Formation was originally Early Paleozoic sediments that were caught in the
collision of Avalonia and Laurentia. The accretion of part of Avalonia and a volcanic
island arc caused the Taconic Orogeny. We saw evidence of this last time, at Port Jervis
NY, in the form of an angular unconformity of the Ordovician Martinsburg Formation
with Silurian Shawangunk sediments. The collision folded early Paleozoic rocks, made a
highlands that supplied the new foreland basin with Green Pond, Shawangunk and High
Falls-Bloomsburg sediments. Around here, it thrust the Wissahickon over the Grenvillian
craton, here represented by the Baltimore Gneiss.
Wissahickon metapelites range from Garnet, through Staurolite, to Kyanite grade. The
Kyanite schists are near migmatites and intrusive granites.
The plan: In the morning we will tour the area between Valley Green Inn and Rex Ave.
After lunch at Valley Green Inn, we will map the area along near Bell's Mill Rd.
Directions: Valley Green can be approached from the Pennsylvania Turnpike exit 333.
Keep right after the toll to Germantown Pike East; continue 5 miles into Chestnut Hill.
Where the downhill on Germantown Av. starts, go to Springfield Ave. Turn right and
follow Springfield Avenue, then take the right fork to Valley Green Road at the edge of
Fairmont Park.
Morning
Once you reach the parking lot near the Valley Green Inn, walk across the bridge and
follow the trail along the Wissahickon to the north. We will return by retracing our steps.
Expect the morning walk, with stops, to take 2 1/2 hours; longer if we take our time.
.
STOP #1: Outcrop of Wissahickon Schist.
The small outcrop at the shelter is Wissahickon Formation. The schists have visible
muscovite crystals showing foliated texture. The mica grains have grown perpendicular
to directed pressure during the collisional event. Also present in the Wissahickon
Formation is quartzite, interlayered with the schist. In the outcrop photos below the
quartzite, which is more resistant to weathering, appears as a smoother often raised
surface.
Mica schists were originally shales deposited in quiet waters; the quartzites were once
sandstones from higher energy environments. The sediments were originally tempestites
and turbidites in a backarc basin. The alternating sands and shales are called "Flysch",
typical of converging margins.
STOP 2: TP conditions for genesis of the abundant garnet schist.
The release of water from clay makes mica. Micas grow so that the plates are oriented
in the rock to minimize differences in pressure, and they regrow if the direction of
pressure changes. This results in foliated layering of micas perpendicular to the directed
pressures present in convergent margins.
The layers in the Wissahickon Formation's schists are not flat because the rocks have
been deformed during and after growth of the mica.
Other minerals present include: Almandine Garnet (dark red brown dodecahedra),
Staurolite, (dark red brown rectangular laths), and Kyanite (light blue to whitish needles)
in order of increasing metamorphic grade.
The minerals Garnet + Staurolite + Kyanite imply temperatures higher than 550° C at
pressures of 4 kbar (about 4000 atmospheres). High pressures indicate great depths
AND/OR great amounts of directed pressure in a collision.
STOP 3: Outcrop of Wissahickon showing large recumbent fold.
Thrust faulting of thick sequences of sediments results in recumbent folds. These Nappes
are typical of fold and thrust terrains such as the Alps. The collision of Laurentias'
Grenville coast and Avalonia folded and faulted the layers of sediment from the ocean
floor between. Because quartzite does not foliate, it is possible see the structure of the
folds better using the quartzite layers.
In a collision rock is under considerable pressure, so a local fluctuation in the pressure
can cause the rock to fold.
Notice that the schist layers are not folded in the same way as the quartzite. The micas
responded to the uneven stress by regrowing with its plates aligned perpendicular to the
directed pressure. This process is continuous; a constant reorientation of mica occurs in
pressurized schist.
STOP 4: Outcrop of migmatized Wissahickon Schist.
This Wissahickon is a migmatite here. The lighter rock is granite with quartz, feldspar
and muscovite; the minerals are not aligned with the layering and are randomly oriented.
Chemical analysis of the granite mineral assemblage shows that it is melted schist. The
phase diagrams of minerals in the schist: biotite, muscovite, garnet, staurolite, and
kyanite, indicate that the schist reached temperatures near 600° C, which is the
experimental melting point of the granite. In some areas near here, small bodies of
granitic magma have coalesced and solidified.
Stop 5 Schist and Quartzites showing conformal folding.
In general, the schist shows the same structure on the large scale.
Stop 6. Around the corner and uphill, the Schist shows more interesting features
Augen Gneiss
Fold Tubes
Small K-spar Pegmatites
Augen form in rocks which have undergone metamorphism and shearing. The core of the augen is a
porphyroblast or porphyroclast of a hard, resilient mineral such as garnet. The augen grows by
crystallisation of a mantle of new mineral around the porphyroblast. The mantle is formed contiguous with
the foliation which is imparted upon the rock, and forms a blanket which tapers off from either side of the
porphyroblast within the strain shadows.
During shearing, the poprhyroblast may rotate, to form a characteristic augen texture of asymmetric
shearing. In this case, the position of the tails is unequal across the foliation, with some augen showing
clear drag folding of the mantle into the strain shadow. This derives a form of shear direction information.
A metamorphic rock which is clotted with augen is often called an augen gneiss.
STOP 7: Layer of amphibolite
Near the top of the hill, overlooking the creek is an amphibolite. The dominant mineral
is hornblende.
Amphibolites are metamorphosed basalt. Are inclusions of Wissahickon Schist within
the metamorphosed basalt present?
Discussion: How would you decide if the original basaltic rock was a horizontal lava
flow, a dike or a sill, part of an ophiolite sequence, etc.
Return to Stop 4, then walk back toward the Inn for a short distance. Your instructor will
show you to the lower path.
STOP 8: The migmatites of stop 4 seen again closer to the stream. Note the stop 4
outcrop above us. This layer continues across the stream.
You look to the northeast from here to find the layer at stop 4, so the rocks strike to the
NE, and dip 60° to the NW.
Continue north along the lower path, keeping an eye on the minerals in the schist. You
are checking to see if the highest grade mineral is almandine garnet, staurolite, or
Kyanite. Wherever a higher grade mineral appears, note the latitude and longitude.
STOP 9. Larger granitic layers crosscutting foliation in the Wissahickon Schist.
Layers of the Wissahickon are broken by the granite, so the granite is younger than the
metamorphism and deformation
Wissahickon Schist ~430 my.
If time permits:
STOP 10: A faulted fold. Using PENCIL, sketch this faulted fold in your notebook. Then
try to reconstruct the original fold with a second sketch, showing the fault plane and
directions of offset.
Return to our vehicle, put your field equipment away, and wash up for lunch at Valley
Green Inn.